Pigment Concentrations In Sediments Research Articles

Weakened pelagic-benthic coupling on an Arctic outflow shelf (Northeast Greenland) suggested by benthic ecosystem changes

Arctic marine ecosystems are becoming more boreal due to climate change. Predictions of ecosystem change focus mainly on Arctic inflow or interior shelves, with few comprehensive studies on Arctic outflow regions. During September–October 2017, soft-bottom communities were sampled and benthic ecosystem processes were quantified at 12 stations on the Northeast Greenland shelf (outflow shelf) and compared to the last regional ecosystem study, conducted in 1992 and 1993. The benthic habitat was characterized in terms of sediment granulometry, pigment concentrations, and porewater chemistry (dissolved inorganic carbon, nutrients). Total abundance and biomass of macrobenthos and meiobenthos, bacterial abundance, porewater dissolved inorganic carbon and ammonium concentrations were higher on the outer shelf compared to locations adjacent to the Nioghalvfjerdsfjorden glacier at 79°N and the inner shelf stations (e.g., macrofauna: 1,964–2,952 vs. 18–1,381 individuals m−2). These results suggest higher benthic production in the outer parts of the NEG shelf. This difference was also pronounced in macrobenthic and meiobenthic community structure, which was driven mainly by food availability (pigments with 1.3–4.3 vs. 0.3–0.9 µg g−1 sediment, higher total organic carbon content and bacterial abundance). Compared to the early 1990s, warmer bottom water temperatures, increased number of sea-ice-free days and lower sediment pigment concentrations in 2017 were accompanied by decreased polychaete and increased nematode abundance and diversity, and a different community structure of nematode genera. The present study confirms previous reports of strong pelagic-benthic coupling on the NEG shelf, but highlights a possible weakening since the early 1990s, with a potential shift in importance from macrofauna to meiofauna in the benthic community. Increasing inflow of Atlantic water and decreasing volume transport and thickness of sea ice through the Fram Strait, probably affecting the Northeast Water Polynya, may be responsible, suggesting ecosystem-wide consequences of continued changes in sea-ice patterns on Arctic shelves.

Annual biogeochemical cycling in intertidal sediments of a restored estuary reveals dependence of N, P, C and Si cycles to temperature and water column properties

Estuarine intertidal sediments are important centres for organic matter remineralization and nutrients recycling. Nevertheless, there is limited understanding regarding how these processes occur along the salinity gradient and their seasonality. Here, we report on the seasonal biogeochemical cycles from three types of intertidal sedimentary habitats (freshwater, brackish and marine) located in the Western Scheldt estuary (The Netherlands and Belgium). A full year of solute fluxes, porewater nutrient and sediment pigment concentrations at a monthly resolution revealed clear differences in the biogeochemistry of the three sites, indicating that environmental conditions determined the local nutrient dynamics. Temperature controlled sediment oxygen consumption rates and nutrient fluxes, but also affected pore water nutrient concentrations up to 14 cm deep. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (−1.62 mmol m−2 d−1 and -2.84 mmol m−2 d−1, respectively), while only the freshwater sediments showed a net influx of phosphate (−0.07 mmol m−2 d−1). We estimated that intertidal sediments remineralized a total of 10,000 t C y−1, with 97% of mineralization occurring in the brackish and marine parts. Overall, sediments removed 11% (1500 t N y−1) and 15% (∼200 t P y−1) of the total nitrogen and phosphorus entering the estuary from riverine input. Moreover, observations revealed the historical improvement of water quality resulting from water treatment policies. This spatiotemporal study of OM remineralization and early diagenesis in estuarine systems highlights the importance of intertidal sediments for estuarine systems. Our observations can be used in models to predict estuarine biogeochemistry or assess climate change scenarios.

Use of sedimentary algal pigment analyses to infer past lake-water total phosphorus concentrations

We tested the feasibility of using sedimentary algal pigment analyses by spectral deconvolution to infer past lake-water total phosphorus concentrations. We established equations that link lake-water nutrient concentrations and sediment pigment concentrations, using a combination of calibration in both space and time, with a training set of 31 Swedish lakes. The calibration dataset yielded a significant positive relationship between total carotenoid concentrations and lake-water total phosphorus concentrations. We also compared sediment-pigment-based nutrient inferences with time series of water column monitoring data to evaluate whether temporal changes in total phosphorus concentrations are well captured by analysis of sedimentary pigments. We found that changes in pigment preservation through time can alter the relationship between concentrations of lake-water nutrients and sedimentary pigments, thus limiting the reliability of historical ecological conditions inferred from pigments in the sediment. Our data suggested that ratios of Chlorophyll derivatives to total carotenoids (CD/TC ratio) and Chlorophyll a to Chlorophyll derivatives (CPI) can be used as proxies for pigment preservation. Using our approach, inferred temporal changes in water-column total phosphorus concentrations in lakes are promising, but require further development, specifically with respect to the influence of pigment degradation in both the water column and sediments, as well as the factors that control such degradation.

Reconstruction of trophic state shifts over the past 90 years in a eutrophicated lake in western Switzerland, inferred from the sedimentary record of photosynthetic pigments

Anthropogenic eutrophication can initiate vast and persistent ecosystem state changes in lakes. Such changes may be characterized by increased phytoplankton taxa variability, which can affect the effectiveness and time of lake recovery mechanisms. Lake Morat in Switzerland has undergone intense eutrophication in the twentieth century (phosphorous concentrations up to 150 μg L−1) caused by excessive nutrient loadings from agricultural intensification and urbanization. Phosphorous reduction measures since 1986, such as the ban of phosphates in detergents and decreased use of fertilizers in agriculture, have resulted in total phosphorous concentrations up to 20 μg L−1 today. Despite this drastic reduction of total phosphorous, total biomass production stays high. We investigate historical changes in the phytoplankton community during the eutrophication and re-oligotrophication periods by comparing historical limnological data with sediment pigment concentrations measured by HPLC and other geochemical proxies in a radiodated sediment core. For the last 90 years, we identified four major trophic state changes in Lake Morat. The first period (AD 1924–1937) is characterized by low pigment concentrations and nutrient inputs, with good oxygen conditions. This section represents trophic conditions before the intensive eutrophication phase. The second period (AD 1937–1970) revealed an abrupt increase in pigment concentrations with higher primary production, cyanobacteria dominance and reduced oxygen levels. Oscillaxanthin indicated a Planktothrix rubescens dominance (AD 1954–1970) with concentrations up to 800 nmol g−1 OM. Their decline after AD 1970 suggests the initiation of an intense eutrophication phase (AD 1970–1983) associated with the dominance of other cyanobacteria species, higher total phosphorous inputs, and intense anoxia. In the restoration period (AD 1983–2014), there was a shift in the phototrophic community from cyanobacteria to green algae dominance, yet some cyanobacteria species remain present. Rapid phytoplankton community changes were identified in the studied period, yet overall primary production response was low. Limnological data revealed a delay of phosphorous reduction due to phosphorous recycling from the sediments. The observation of complex lake ecosystem reactions to prolonged eutrophication and subsequent re-oligotrophication, as shown by the paleolimnological and limnological data in this study, emphasize the importance of careful lake management to revert eutrophication back to historical reference biomass values.

Ecological changes in two contrasting lakes associated with human activity and dust transport in western Wyoming

The atmospheric transport and deposition of aerosols has the potential to influence the chemistry and biology of oligotrophic alpine lakes. In recent decades, dust and nitrogen emissions to alpine ecosystems have increased across large areas of the western U.S., including Wyoming. Here, we use sediment geochemistry and 87Sr/86Sr and 143Nd/144Nd isotopes to examine historical dust deposition rates to alpine lakes in the southwestern region of the Wind River Range, Wyoming. We evaluate the biological response using diatom fossil assemblages and sediment pigment concentrations. Sediment core analyses indicated that prior to a recent rise in dust flux, phosphorus concentrations and species composition were similar to those found in other alpine lakes in the region. Concomitant with a ∼50 fold increase in dust flux to the sediments circa 1940, sediment proxies revealed a two- to threefold increase in normalized sediment phosphorus content, an increase in the diatom-inferred total dissolved phosphorus concentration from ∼ 4 to 9-12 μg L−1, a tenfold increase in diatom production, and a relative increase in cyanobacteria abundance. The increase in dust influx during the 20th century appears to be due in part to human factors and demonstrates the potential for dust and other atmospheric pollutants to significantly alter remote aquatic ecosystems.

Composition, abundance and age of total organic carbon in surface sediments from the inner shelf of the East China Sea

Surface sediments were collected from the inner shelf of the highly dynamic East China Sea (ECS) in December 2009 and November 2010. In between the two cruises, a big flood event occurred in July, 2010. Elemental, stable C, N isotope, radiocarbon isotope (∆14C), black carbon (BC) analysis, and chemical biomarkers were utilized to study the effect of the sources, short-term events and physical processes on the composition, abundance, and age of the surface sedimentary OC in this dynamic system. Bulk N/C ratio of 0.10 to 0.14 and δ13C of −23.1‰ to −20.9‰ in these samples indicated a mixture source of terrestrial, deltaic and marine derived OC during both cruises. The TOC was 1,980 to 5,040yr old based on ∆14C, and BC extracted from two 2009 sediment samples was older: 5,480 and 5,920yr old. While the Changjiang River was a main source of the terrestrial derived OC, refractory BC and reworked marine organic matter seemed to comprise the older, less reactive carbon pool in the surface sediments of the ECS. Monte Carlo simulations indicated that more than 50% of the TOC was derived from the marine end-member, followed by the deltaic OC. The chlorophyll a of the 2010 sediment samples (11.06±10.15nmolg−1 OC, n=7) was significantly lower than that of the 2009 samples (34.58±20.29nmolg−1 OC, n=7). However, zeaxanthin, a biomarker for cyanobacteria, had significantly higher concentration in the 2010 sediment samples (9.26±3.48nmolg−1 OC, n=7) than that of the 2009 samples (5.50±4.62nmolg−1 OC, n=7). The lack of benthic–pelagic coupling of pigment concentrations in surface sediments after the flood of 2010 was likely due to lateral transport of surface sediments to and from this region. While there were no significant linkages between in situ shelf-derived phytoplankton sources of OC and surface sediment pigment concentrations in 2010, other algal inputs from local deltaic lakes may have contributed to the compositional changes in concentration of sediment carotenoids. The flood of summer 2010 likely played a critical role in the observed shift of the locations for the highest terrestrial biomarker concentrations in sediments. Little evidence remained from the 2010 flood event, which suggested that winter wind/wave energy and hydrodynamic sorting had a substantial effect on sediment OC redistribution.

An environmental record of changes in sedimentary organic matter from Lake Sattal in Kumaun Himalayas, India

Sattal a small mountainous lake in the Kumaun Himalayas has been impacted by various cultural activities in recent years. We explored the effects of human-induced changes in this lake by using various geochemical proxies. Shifts in TOC and N flux, C/N ratio, stable isotopes ( δ 13C and δ 15N), n-alkane, and pigment concentrations in sediments indicate a steady increase in primary productivity over the last few decades. The trophic status of the lake has changed from mesotrophic to eutrophic condition. The C/N, CPI, and TAR based ratios in sediments indicate accumulation of algal matter derived primarily from in situ production, with limited input of terrestrial organic matter from the watershed. The low (between 0.1 and 1‰) δ 15N values imply N 2-fixation by cyanobacteria, and the decrease in δ 13C values up-core represent the effect of sewage input and land based runoff, or possible contribution from microbial biomass. The pigments change from non-N 2 fixing cyanobacterial species to the N 2-fixing community, and are consistent with the proxy-based productivity shifts inferred in the lake. The deeper sediments are affected by post-diagenetic changes causing an increase in δ 13C (and possibly δ 15N) due to mineralization of organic C and N.

Sedimentary pigments on the Pakistan margin: Controlling factors and organic matter dynamics

Abstract Sedimentary pigments can provide information on various aspects of benthic processes and biogeochemistry. In this study, sediments from sites at depths of 140 to 1850 m spanning the Pakistan margin oxygen minimum zone (OMZ), and representing dramatic contrasts in depositional conditions and benthic communities, were analysed for pigment yields and compositions. This has allowed a rare consideration of how different factors, including oxygen concentration, organic matter (OM) supply and biological activity (e.g., degree of bioturbation) may influence sedimentary pigment distribution in a continental margin environment. It has also allowed one of the first efforts to study the impact of biological activity on pigment concentrations in the natural environment, rather than in a microcosm setting. Total extractable surface sediment pigment concentrations showed a range of ∼1.5–49 μg g −1 of dry sediment. The pigment suite was dominated by pheophytin and pheophorbide (which together constituted ∼75% of total pigments), with minor amounts of chlorophyll- a , alloxanthin, diatoxanthin, zeaxanthin and β-carotene. Low oxygen concentration appeared to influence pigment abundance by increasing the concentration of refractory pigments ultimately buried. In addition, maximal pigment abundances were associated with minimal amounts of sediment mixing by macrofauna, and at one site macrofaunal digestion may have altered the relative dominance of the different pheopigments. In contrast, the lack of a marked increase in pigment concentrations after the summer monsoon plus the lack of a correlation between pigment concentration and water depth suggest that OM supply plays a relatively small role in controlling sedimentary pigments. Pigment concentrations and suites, together with modelled decay half-lives (∼0.8–15 years), suggest that in general OM on the Pakistan margin, despite the low oxygen concentrations and monsoon-driven productivity, is surprisingly degraded compared to that on other continental margins with OM-rich sediments. As an exception to this general situation, pigment suites appeared to be relatively “fresh” (i.e., contained more labile components) compared to those found on the Oman margin of the Arabian Sea, but overall concentrations were lower. This is consistent with the higher oxygen concentrations, productivity and macrofaunal abundances found on the western margin of the Arabian Sea. Principle component analysis of pigment suites showed that they correlated with an amino-acid-based degradation index (DI). Thus, the pigment composition of marine sediments is related to, and may serve as an indicator of, the state of decay of sedimentary OM.

Preservation conditions and the use of sediment pigments as a tool for recent ecological reconstruction in four Northern European estuaries

Down-core sediment pigment concentrations from four Northern European estuaries were measured using high-performance liquid chromatography (HPLC) to investigate phytoplankton community structure and preservation conditions over the last ca. 100 years where all sites have experienced different levels of eutrophication. Phytoplankton pigments have been shown to be useful biomarkers for phytoplankton community structure and abundance due to their taxonomic specificity. The pigment concentrations and sediment pigment inventory showed large variation between the four sites. Concentrations ranged from more than 6000 nmol/g OC to less than 100 nmol/g OC and the inventory integrated over the top 10 cm from more than 300 nmol/cm 2 to less than 30 nmol/cm 2 for total identified pigments. Good pigment preservation in Mariager Fjord (Denmark) reflected the almost permanently anoxic conditions. Pigments in Laajalahti (Finland) showed peak concentrations around the time of highest nitrogen loading events known from historical and modelled records over the past 100 years. In contrast, poor down-core preservation of pigments (especially carotenoids) was observed in the Ems-Dollard (The Netherlands) and Himmerfjärden (Sweden) estuaries. The Ems-Dollard site is an intertidal mudflat that experiences daily exposure to light and air, which enhances pigment degradation. In Himmerfjärden, resuspension is an important process affecting both the sedimentation rate and degradation properties. The different preservation conditions at the four sites were supported by the differences in two degradation indicators; the ratio of pheopigment- a to chlorophyll- a and total carotenoids to total pigments. Class-specific carotenoid pigments represented the dominant algal groups reported from each site, however, no distinct down-core changes in the pigment composition were observed at any of the four sites. This indicated that changes in plankton community structure on the group level have been limited over this time period or masked by low preservation of pigments.

Derivative analysis of absorption features in hyperspectral remote sensing data of carbonate sediments

This study uses derivative spectroscopy to assess qualitative and quantitative information regarding seafloor types that can be extracted from hyperspectral remote sensing reflectance signals. Carbonate sediments with variable concentrations of microbial pigments were used as a model system. Reflectance signals measured directly over sediment bottoms were compared with remotely sensed data from the same sites collected using an airborne sensor. Absorption features associated with accessory pigments in the sediments were lost to the water column. However major sediment pigments, chlorophyll a and fucoxanthin, were identified in the remote sensing spectra and showed quantitative correlation with sediment pigment concentrations. Derivative spectra were also used to create a simple bathymetric algorithm.

Carotenoid Pigments as Tracers of Cyanobacterial Blooms in Recent and Post-glacial Sediments of the Baltic Sea

The distributions of carotenoid pigments were studied in recent and postglacial sediments collected in the Gotland Basin, in the northern part of the Baltic proper, and in the eastern Gulf of Finland in May 1999. The aim was to provide a systematic, historical review of the occurrence and intensity of cyanobacterial blooms in the Baltic Sea in recent centuries. The presence of pigments was determined using high-pressure liquid chromatogrpahy (HPLC), with attention mainly focused on pigments considered as markers for cyanobacteria. The pigment concentrations in sediments from the 3 sampling locations were found to differ markedly. In general, concentrations were highest in sediments from the Gotland Basin, and lowest in sediments from the eastern Gulf of Finland. In all 3 cores echinenone was the dominant carotenoid in the topmost layer of the sediment (0-1 cm). In the deeper sections of the cores, myxoxanthophyll and zeaxanthin dominated. To our knowledge, this is first time that myxoxanthophyll and echinenone have been detected in Baltic Sea sediments from the early Litorina stage. The decrease in the pigment content with sediment depth coincides with a decrease in carbon content, and also fits in with general historical records of the occurrence and intensity of cyanobacterial blooms in the Baltic Sea, which show that cyanobacterial blooms were seldom recorded before World War II.

Macrobenthic community structure within and beneath the oxygen minimum zone, NW Arabian Sea

Investigations of macrobenthos were carried out within and beneath the oxygen minimum zone (OMZ, <0.5 ml l −1) during Fall 1994 on the Oman margin, NW Arabian Sea. Six stations (400, 700, 850, 1000, 1250 and 3400 m) were characterized with respect to macrofaunal abundance, biomass, body size, taxonomic composition, diversity and lifestyles, and the relation of these parameters to environmental conditions. The OMZ (400–1000 m) was dominated by a dense (5818–19,183 ind m −2), soft-bodied assemblage consisting largely (86–99%) of surface-feeding polychaetes. Spionids and cirratulids dominated at the 400- and 700-m stations, paraonids and ampharetids at the 850- and 1000-m stations. Molluscs and most crustaceans were common only below the OMZ (⩾1250 m); a species of the amphipod Ampelisca was abundant within the OMZ, however. Both density and biomass were elevated within the OMZ relative to stations below but body size did not differ significantly among stations. The lower OMZ boundary (0.5 ml l −1) was not a zone of enhanced macrofaunal standing stock, as originally hypothesized. However, abundance maxima at 700–850 m may reflect an oxygen threshold (0.15–0.20 ml l −1) above which macrofauna take advantage of organically enriched sediments. Incidence of burrowing and subsurface-deposit feeding increased below the OMZ. Species richness ( E[ S 100]), diversity ( H′) and evenness ( J′) were lower and dominance (R1D) was higher within than beneath the OMZ. Within-station (between-boxcore) faunal heterogeneity increased markedly below the OMZ. Surface sediment pigment concentrations and oxygen together explained 96–99% of the variance in measures of E[S 100], H′ and J′ across the transect; grain size and % TOC did not yield significant regressions. Pigments, assumed to reflect food availability and possibly oxygen effects on organic matter preservation, were negatively correlated with species richness and evenness, and positively correlated with dominance. The reverse was true for water depth. Macrobenthic patterns of calcification and lifestyle within the Oman margin OMZ (0.13–0.3 ml l −1) match the dysaerobic biofacies of paleo-environmental reconstruction models.

The spring bloom and its impact on benthic mineralisation rates in western Irish Sea sediments

The impact of the spring bloom on benthic remineralisation rates was studied in offshore waters of the western Irish Sea during 1998. Initiation of the spring bloom coincided with the onset of thermal stratification at the end of April. Peak chlorophyll biomass (6.48 mg chl m -3 ) and algal standing stock (145.0 mg chl m -2 ) were measured on May 11, and the bloom lasted approximately 1 mo. Sediment oxygen uptake increased to 1658 μmol m -2 h -1 for a short period in early May, which was followed by increased denitrification (up to 48 μmol N m -2 h -1 ) in late May and early July. Subsequently sulphate reduction increased (up to 83 μmol SO 4 2- m -2 h -1 ) in early July. Efflux of nitrate remained low but constant (10 μmol m -2 h -1 ) throughout the study, accounting for 27 % of ammonified organic N, with the remaining 63% being denitrified. Sediment depth profiles of chlorophyll and phaeopigment showed deep (>30 cm) mixing of phytodetritus into the sediment in early July, which may explain the high rates of sulphate reduction measured at this time. Evidence of a large phytodetrital flux to the benthos during spring 1998 was limited. Deepening of chlorophyll isopleths suggested sinking of algae in early May, although sediment pigment concentrations indicated a continual but low input. Benthic oxygen consumption represented 46 % of total spring phytoplankton production or 61% of new production and approximately balanced the calculated flux of detrital carbon (C) to the benthos. The impact of detrital C (assumed to be largely phytodetritus immediately after the spring bloom) on the benthos was short lived, being rapidly remineralised, and in turn little spring production would have been available for secondary production, e.g. Nephrops norvegica. In such years, benthic production, particularly that of N. norvegica, must be supported by phytoplankton production which takes place after the bloom or by detrital C which is advected into the area following the breakdown of the gyre.

Temporal changes in the biochemical composition and nutritional value of the particulate organic matter available to surface deposit-feeders:a two year study

Gross sedimentation rates (GSR) were monitored together with the main characteristics of the collected matenal (i.e. organic content, C, N, total proteins, lipids, carbohydrates, available proteins and amino acids) over a 2 yr cycle at a shallow station (18 m) of the bay of Banyuls, France. In addition, sediment pigment concentrations and meiofaunal densit~es were recorded monthly during a 1 yr period. GSR ranged between 0.6 and 317.8 g DW m ' cl'' Spring and summer were characterized by relatively low and constant GSR whereas fall and wlnter were characterized by relat~vely high and hlghly variable GSR. There was a negatlve relationship between GSR and the organic content of material collected within the sediment traps, suggesting the importance of resuspension in controlling GSR. Growth rates of the deposit-feeding bivalve Abra ovata fed sediment trap material collected on 6 sampling dates were also measured. Significant changes in important nutrient components (e.g. available proteins) of sedimenting materials from these different sampling dates correlated with growth differences and confirmed the existence of temporal changes in the quality of the material collected in the sediment traps. The highest growth rate was obtained for the material collected during May 1993, which coincided with maximal meiofauna densities and maximal pigment concentrations at the surface of the sediment. The best descript.ion of growth was obtained when using available proteins and total lipids as the independent variables of simple linear regression models. This supports the use of these parameters as descriptors of food quality. The consequences of our results on the parameterization of changes in food quality within models of continental shelf food webs are discussed. It is suggested that such changes should be simulated through control functions based on available protein contents.

The influence of the filter-feeding bivalve Cerastoderma edule L. on microphytobenthos: a laboratory study

The effect of the filter-feeding bivalve Cerastoderma edule L. on microphytobenthos was investigated in three laboratory experiments. In the first experiment adult specimens of C. edule placed in flow-through aquaria containing sieved sediment were compared to aquaria without C. edule. The aquaria were incubated for 40 days, and sampled weekly for inorganic nutrient concentrations in the overlying water, sediment pigment concentrations and determination of benthic primary production. The presence of C. edule significantly increased the release of NH 4 + and PO 4 3− to the overlying water. The chlorophyll α concentration in C. edule treatments increased, and was significantly higher than in controls. The benthic primary production in C. edule treatments increased, reaching a mean maximum of 27.9 mg C·M −2·h −1, while the controls stayed at the initial mean level of 14.7 mg C·m −2·h −1 throughout the experiment. When the possible effect of fertilizing with faecal pellets from C. edule was tested in a second experiment, no differences were observed in nutrient fluxes, chlorophyll α concentration or primary production. In a third experiment, the effect of nutrients released by C. edule was investigated. Water from an aquarium containing a number of C. edule equivalent to the high natural density of cockles at the site of collection of the sediment was directed to four of the aquaria. The primary production was significantly higher in C. edule treatments than in controls. Thus the stimulating effect of C. edule on the microphytobenthos seems to be mediated by the release of nutrients, mainly NH 4 +, by the cockles themselves rather than through the deposition of faecal pellets.

Intertidal and Shoal Benthic Community Metabolism in a Temperate Estuary: Studies of Spatial and Temporal Scales of Variability

Variability in benthic community metabolism, sediment chlorophyll and sediment organic carbon attributable to substrate type and season were examined. Substrate types included intertidalSpartina alterniflora marsh, mudflat, and sandflat and subtidal eelgrass (Zostera marina) and bare sand bottoms. Significant differences in parameter estimates were found among the different substrate types for each specific sampling data. When estimates were pooled over the entire study period, significant differences were found only for respiration rate and sediment pigment concentrations. The data indicate no obvious ranking in magnitude for any parameter that can be attributed to substrate type or season. Small-scale spatial variability and rapid temporal changes (turnover) are postulated as the principal reasons governing the overall results.